Been there tried that with the NEF.
That twist is fast and you would need a very heavy (or long) bullet to stabilize well in the barrel (I calculated upward of 900gr). I shot up to a 515
Rem cases are probably the best to work with as they seem to give the best fit and roll crimp well. They also gave higher velocities and lower deviation. I believe that the groove was about .625 or so in the NEF barrel and the Magtechs are too thin. The proper twist seems to be about a 60" or so with reasonable 400-500gr slugs in the 20 if they were a standard conical design. Rem SP20 wads have a very wide base and act as a good seal
Tom
